Why does increased temperature increase resistance

Only some electrons are free to migrate however. Others within each atom are held so tightly to their particular atom that even an electric field will not dislodge them. The current flowing in the material is therefore due to the movement of "free electrons" and the number of free electrons within any material compared with those tightly bound to their atoms is what governs whether a material is a good conductor many free electrons or a good insulator hardly any free electrons.

The effect of heat on the atomic structure of a material is to make the atoms vibrate, and the higher the temperature the more violently the atoms vibrate. In a conductor, which already has a large number of free electrons flowing through it, the vibration of the atoms causes many collisions between the free electrons and the captive electrons.

Each collision uses up some energy from the free electron and is the basic cause of resistance. The more the atoms jostle around in the material, the more collisions are caused and hence the greater the resistance to current flow. In an insulator however, there is a slightly different situation.

There are so few free electrons that hardly any current can flow. Almost all the electrons are tightly bound within their particular atom. Heating an insulating material vibrates the atoms, and if heated sufficiently, the atoms vibrate violently enough to actually shake some of their captive electrons free, creating free electrons to become carriers of current.

This is illustrated in the tutorial above. A circuit connects a 3V Lamp with Iron Wire to two 1. The circuit also contains a Ceramic Spool around which a section of the wire is wrapped. Underneath the spool is a Bunsen Burner to heat the wire. Click the Turn On button to light the Bunsen burner. As the wire heats, the lamp dims, then ceases to glow. The more scattering, the higher the resistance.

Where does this idea belong? Nichrome was invented in , which made electric toasters possible. The general rule is resistivity increases with increasing temperature in conductors and decreases with increasing temperature in insulators. Unfortunately there is no simple mathematical function to describe these relationships. The temperature dependence of resistivity or its reciprocal, conductivity can only be understood with quantum mechanics.